Gollum is a fictional character in J. R. R. Tolkien's fantasy world of Middle-earth. He was introduced in the 1937 fantasy novel The Hobbit, and became important in its sequel, The Lord of the Rings. Gollum was a Stoor Hobbit of the River-folk who lived near the Gladden Fields. In The Lord of the Rings, it is stated that he was originally known as Sméagol, corrupted by the One Ring, and later named Gollum after his habit of making "a horrible swallowing noise in his throat".

Sméagol obtained the Ring by murdering his relative Déagol, who found it in the River Anduin. Gollum called the Ring "my precious", and it extended his life far beyond natural limits. Centuries of the Ring's influence twisted Gollum's body and mind, and, by the time of the novels, he "loved and hated [the Ring], as he loved and hated himself." Throughout the story, Gollum was torn between his lust for the Ring and his desire to be free of it. Bilbo Baggins found the Ring and took it for his own, and Gollum afterwards pursued it for the rest of his life. Gollum finally seized the Ring from Frodo Baggins at the Cracks of Doom in Mount Doom in Mordor, but he fell into the fires of the volcano, where he was killed and the Ring destroyed.

Commentators have treated Gollum as a monster: the Tolkien scholar Verlyn Flieger suggests that Gollum is Tolkien's central monster-figure. Others have described Gollum as a psychological shadow figure for Frodo, and as an evil guide in contrast to the wizard Gandalf, the good guide. They have noted, too, that Gollum is not wholly evil, and that he has a part to play in the will of Eru Iluvatar, the omnipotent god of Middle-earth, necessary to the destruction of the Ring. For Gollum's literary origins, scholars have compared Gollum to the shrivelled hag Gagool in Rider Haggard's 1885 novel King Solomon's Mines and to the subterranean Morlocks in H. G. Wells's 1895 novel The Time Machine.

Gollum was voiced by Brother Theodore in Rankin-Bass's animated adaptations of The Hobbit and Return of the King, and by Peter Woodthorpe in Ralph Bakshi's animated film version and the BBC's 1981 radio adaptation of The Lord of the Rings. He was portrayed through motion capture by Andy Serkis in Peter Jackson's Lord of the Rings and The Hobbit film trilogies. The "Gollum and Sméagol" scene in The Two Towers directly represents Gollum's split personality as a pair of entities. This has been called "perhaps the most celebrated scene in the entire film".

The Tolkien scholar Douglas A. Anderson, editor of The Annotated Hobbit, suggests that Tolkien derived the name "Gollum" from Old Norse gull/goll, meaning 'gold'; this has the dative form gollum, which can mean 'treasure'. Another suggestion is that it derives from golem, a being in Jewish folklore.

In Appendix F of The Lord of the Rings, Tolkien pretends that the name "Sméagol" is a "translation" of the Middle-earth name Trahald (having to do with the idea of "burrowing"), and rendered with a name based on Old English smygel of similar meaning.

The rhyming name of his relative "Déagol" is from Old English: dēagol, meaning 'secretive, hidden'. In Tolkien's Red Book of Westmarch, the name "Déagol" is again supposedly a translation of the "original" name in the author-invented language of Westron, Nahald.

Gollum was introduced in The Hobbit as "a small, slimy creature" who lived on a small island in an underground lake at the roots of the Misty Mountains. He survived on cave fish, which he caught from a small boat, and small goblins who strayed too far from the stronghold of the Great Goblin. Over the years, his eyes adapted to the dark and became "lamp-like", shining with a sickly pale light.